1. Field of the Invention
The invention relates to a self-boosting friction brake, in particular for automobiles. The invention also relates to a apparatus for measuring the coefficient of friction for a friction brake and to a method for regulating a braking force.
2. Description of the Prior Art
One example of self-boosting friction brakes is drum brakes with one or more primary brake shoes. This example is meant to make it clear that the discussion that follows is not limited to disk brakes, although the discussion is directed to disk brakes, which are preferred according to the invention. To attain self-boosting in disk brakes, actuating devices are used that have a wedge mechanism, lever mechanism, or ramp mechanism. At least in theory, an analogy can be made between a lever mechanism and the primary brake shoe of a drum brake.
Known self-boosting disk brakes have a friction brake lining, which to generate a braking force can be pressed with a contact-pressure element against a rotatable brake body; in the case of a disk brake, the brake body is a brake disk. The contact-pressure element is for instance a wedge element, which is movable in a direction of rotation of the brake body and is braced on a buttress at a support angle to a normal of the brake body. The support angle, in the case where a wedge element is used as the contact-pressure element, is its wedge angle; if a lever is used, it is the angle at which the lever is oriented to a normal of the brake body. If a ramp mechanism is used, the buttress is a face or guide (ramp) extending obliquely to the brake disk, and the angle at which the buttress extends relative to the brake body is the support angle. If a ramp mechanism is used, the contact-pressure element can be a wedge element; that is, wedge and ramp mechanisms do not preclude one another. The support angle of the ramp can be constant or can vary over a length of the ramp. In the case of a lever, the support angle changes upon actuation of the brake.
To attain self-boosting, the contact-pressure element is disposed such that when the brake is actuated, a frictional force exerted on the friction brake lining by the rotating brake body urges the contact-pressure element in the direction of an increasingly strong contact pressure exerted on the friction brake lining by the contact-pressure element. This means that a wedge element as the contact-pressure element is moved into a narrower and narrower wedge gap between the buttress and the brake body. A lever as a contact-pressure element is urged in the direction of a smaller and smaller support angle to the normal of the brake body; that is, the lever is disposed in the form of a so-called pushed lever or lever subjected to compressive stress.
Known friction brakes have an actuating device, with which for actuation the contact-pressure element is movable in the direction of rotation of the brake body, while for releasing the brake it is movable counter to the direction of rotation of the brake body. As a result of the motion in the direction of rotation of the brake body, the contact-pressure element braced on the buttress moves toward the brake body and presses the friction brake lining against the brake body.
One example of this kind of self-boosting friction brake is disclosed in European Patent Disclosure EP 953 785. This friction brake is embodied as a full disk brake and has a number of wedge elements, which are disposed on a circular-annular disk and are braced on rollers as buttresses. By rotation of the circular-annular disk, the wedge elements are moved in the direction of rotation of the brake disk and press a number of friction brake linings against the brake disk. By reverse rotation of the circular-annular disk, the known disk brake is released.
In all the known self-boosting friction brakes, the support angle or wedge angle is selected to be large enough that self-locking of the brake is precluded with certainty. In self-locking, blocking of the brake body occurs because of the frictional force exerted on the contact-pressure element by the rotating brake body when the brake is actuated; this frictional force displaces the contact-pressure element in the direction of an increasingly strong contact pressure. The contact pressure of the friction brake lining against the brake body is increased, without increasing the actuating force with which the contact-pressure element is urged in the direction of rotation of the brake body. The contact pressure increases on its own, until self-locking ensues; that is, the brake body blocks. Since a coefficient of friction between the friction brake lining and the brake body changes as a result of such interfering factors as soiling, moisture, water, temperature, and contact pressure, a sufficiently large support angle must be selected to preclude self-locking of the brake with certainty under all possible operating conditions. However, as a result the magnitude of the self-boosting is limited, and despite the self-boosting, a strong actuating force is required. This necessitates a sufficiently powerfully dimensioned actuating device, which in turn leads to high weight and high inertia of masses in motion of the actuating device, with correspondingly worse dynamics. High actuating energy is also required.
Self-locking must always be expected if the so-called brake parameter C* undergoes a change of sign, i.e. from positive to negative or vice versa. The brake parameter is the ratio between the circumferential force generated at a brake body and the associated actuating force of the friction brake. At the point where the change of sign occurs, the brake parameter C* has a pole (or so-called pole point), and at this point the self-boosting of the friction brake tends toward infinity. For the friction brakes under consideration here, the brake parameter C* is affected not only by the support angle α but essentially by the coefficient of friction μ between the friction brake lining and the brake body. For the pole point, the applicable equation is μ=tan α. For μ<tan α, the contact-pressure element must be acted upon by an actuating force to generate a braking action. For μ>tan α, the contact-pressure element is slaved by the rotating brake body as a result of friction, without any actuating force being exerted. This can lead to self-locking of the brake.